1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to the cell structure of anon-volatile memory device.
2. Description of the Related Art
Semiconductor memory devices can be classified into random access memories (RAMs) and read only memories (ROMs). When an interruption in power supply occurs, data stored in a RAM is erased, while a ROM maintains its stored data. For this reason, ROMs are classified as “nonvolatile memory devices.” An erasable programmable read-only memory (EPROM) device is a particular type of nonvolatile memory devices, which is reusable by allowing its stored data to be erased. One variation of EPROM devices is a flash memory device, which allows data to be erased in blocks (two or more bytes) rather than one byte at a time.
Data stored in the EPROM device can be erased by applying ultraviolet rays to a window attached to the surface of a memory chip. Research is being conducted to develop system-on-chip (SOC) devices, which include EPROM devices along with various devices, such as logic circuits or drive circuits. The EPROM devices can be used while the SOC devices are being manufactured, for example, to calibrate disparity between devices or represent manufacturers' names on fabricated chips.
As the line widths of circuits have become finer, the fabrication processes of semiconductors increasingly use silicide layers, such as TiSix, CoSix, and NiSix, to reduce the resistances of gate lines and metal contacts. When a silicide layer is used, a nitride layer is applied as a gate spacer for the transistor, rather than an oxide layer. This improves the characteristics of the transistor by preventing a short between the gate and either the source or drain during the silicidation process.
When a cell of the EPROM device is formed by a process using a nitride layer as the gate spacer, an ONO (oxide-nitride-oxide) layer is formed on top of a floating gate, and a nitride spacer is formed on the sides of the floating gate. That is, the process forms nitride layers all around the floating gate. In such a case, it becomes difficult to erase charges of the floating gate by ultraviolet rays, since ultraviolet layers cannot pass through the nitride layers.